Set proper content type for logs requests

assets/scripts/app/models/artifact.coffee

@@ -24,6 +24,7 @@ require 'travis/model'
     self = this
     Travis.ajax.ajax "/jobs/#{@get('job.id')}/log.txt", 'GET',
       dataType: 'text'
+      contentType: 'text/plain'
       success: (data) ->
         self.set 'body', data
         self.set 'isLoaded', true

assets/scripts/lib/travis/ajax.coffee

@@ -22,7 +22,7 @@ jQuery.support.cors = true
     options.url = "#{endpoint}#{url}"
     options.type = method
     options.dataType = options.dataType || 'json'
-    options.contentType = 'application/json; charset=utf-8'
+    options.contentType = options.contentType || 'application/json; charset=utf-8'
     options.context = this
 
     if options.data && method != 'GET' && method != 'get'